Development Mode (aka "devMode")

Struts 2 has a setting (which can be set to true or false in struts.properties) called devMode (= development mode). When this setting is enabled, Struts 2 will provide additional logging and debug information, which can significantly speed up development.

You can also set this constant in your struts.xml file: <constant name="struts.devMode" value="true" />.

What does it do?

When enabled, Struts 2 will reload your resource bundles on every request (meaning you can change your .properties files, save them, and see the changes reflected on the next request).
Note: this option can also be set standalone via struts.i18n.reload = true

It will also reload your xml configuration files (struts.xml), your validation files, and so on, on every request. This is useful for testing or fine-tuning your configuration without having to redeploy your application every time.
Note: this option can also be set standalone via struts.configuration.xml.reload = true

And thirdly, perhaps the setting which is less widely known, and therefore a source of much confusion: it will raise the level of debug or normally ignorable problems to errors. For example: when you submit a field which cannot be set on an action 'someUnknownField', it will normally be ignored. However, when you're in development mode, an exception will be thrown, telling you an invalid field was submitted. This is very useful for debugging or testing large forms, but can also be confusing if you're relying on parameters in your request that are not set on the action, but which you are using directly in your view layer (warning: bad practice, you should always validate input from the web).

Don't forget...

By default, the development mode is disabled, because it has a significant impact on performance, since the entire configuration will be reloaded on every request.

Page rendering is slow

If you experience slow page rendering when devMode is on it's mostly because Freemarker cache is disabled during devMode. You can explicit enable cache and any other options disabled by devMode, see example below: